The present invention relates to a liquid crystal device and to a display incorporating such a device. Such a display may, for example, be used in the fields of television, computer monitors, flat screen devices and image processing devices.
Liquid crystals are materials characterised in part by molecules of anisotropic (i.e. non-spherical) shape. In liquids, molecules lack ordering. In crystals, molecules are both orientationally and positionally ordered. In liquid crystals, molecules show ordering which is intermediate between that of a liquid and that of a true crystal. A number of different liquid crystals phases exist. The nematic liquid crystal phase is characterised by anisotropic molecules showing some degree of orientational alignment, but lacking relative positional order. In smectic liquid crystal phases, molecules show some orientional order and also some positional order but in a limited number of spatial directions.
The actual phase occuring for a liquid crystal material depends on temperature. At high temperatures (approximately above 100xc2x0 C. for many materials used in display devices) all order is lost and the material becomes an isotropic liquid. As temperature is reduced materials commonly used in display devices undergo a transition to a nematic phase. As temperature reduces still further, a transition to a smectic phase or to a true crystal phase can occur at low temperature.
A known type of display, for example as disclosed in U.S. Pat. Nos. 4,566,758 and 4,635,051, comprises a xe2x80x9cpi-cellxe2x80x9d. A layer of nematic liquid crystal is disposed between alignment layers which provide parallel alignment and low pre-tilt, typically less than 10xc2x0. The nematic liquid crystal has positive dielectric anisotropy. The state of the liquid crystal directors in the layer is controlled by an electrode structure, for example of pixelated active matrix addressing type
In such a liquid crystal display, each pixel is operated over a predetermined voltage range. The liquid crystal director in the central region of the liquid crystal layer remains substantially perpendicular to the plane of the alignment layers at all voltages within the range of operation, whereas the liquid crystal director in the surface regions close to the alignment layers undergoes reorientation as the applied voltage is varied. Such a device is known as a xe2x80x9csurface mode devicexe2x80x9d because switching occurs in the surface regions adjacent to the alignment layers whereas little or no switching occurs in the bulk of the liquid crystal.
Because, as is well known, liquid crystals are optically anisotropic (xe2x80x9cbirefringentxe2x80x9d), a polarised beam of light passing through a liquid crystal will in general suffer some change in its polarisation state. Use of a liquid crystal in conjunction with optical polarisers therefore allows for the construction of an optical shutter, which in turn can form the basis of an optical display system. For a surface mode device, the thickness of the liquid crystal layer is typically chosen such that the optical retardation of the system varies by a half wave over the range of operating voltage of the device in the case of a transmissive display, or by a quarter wave in the case of a reflection mode device.
Another known type of surface mode device is disclosed in WO 97/12275. In this device, a negative dielectric anisotropy nematic liquid crystal layer is disposed between alignment layers which provide a very high pre-tilt, typically greater than 80xc2x0. The liquid crystal in the bulk of the layer is aligned so that the liquid crystal directors are substantially parallel to the alignment layers. Switching of the liquid crystal director occurs predominantly in the near-surface regions of the device when the applied field is switched between two predetermined values. The variation of the applied field causes a variation in the amount of splay distortion of the director in the near-surface regions. Such a device thus operates as a variable retarder as in the case of the pi-cell.
According to a first aspect of the invention, there is provided a surface mode liquid crystal device comprising a layer of nematic liquid crystal having viscosity coefficients xcex71, xcex72 and xcex31 such that (xcex71xe2x88x92xcex72)/xcex31xe2x89xa71.15 or (xcex71xe2x88x92xcex72)/xcex3xe2x89xa60.9.
According to a second aspect of the invention, there is provided a liquid crystal device comprising a layer of nematic liquid crystal having viscosity coefficients xcex71, xcex72 and xcex31 such that (xcex71xe2x88x92xcex72)/xcex31xe2x89xa71.15 or (xcex71xe2x88x92xcex72)/xcex31 xe2x89xa60.9 at a temperature such that the liquid crystal is at least 5xc2x0 C. away from a transition to another phase.
The other phase may be a smectic phase.
The liquid crystal may show a nematic phase at at least one temperature in the range of 0-60xc2x0 C.
Unlike conventional liquids whose viscosity at a given temperature is determined by a single number, the description of the dynamic behaviour of a nematic liquid crystal requires that five viscosity coefficients be specified. These viscosity coefficients are generally referred to as xcex71, xcex72, xcex73, xcex712 and xcex31 and are described in F. M. Leslie, Quart. J. Mech. Appl. Math 19, pp. 357 (1966); F. M. Leslie, Arch. Ratio. Mech. Anal. 28, pp. 265 (1968); and M. Miesowicz Bull. Inten. Acad. Polon. Ser. A, 228, 1936, the contents of which are incorporated herein by reference. A physical understanding of the nature of the viscosities xcex71, xcex72, xcex73, xcex712, xcex31 can be gained with reference to FIG. 1 of the accompanying drawings.
Consider an idealised experiment in which a uniformly aligned nematic lies between two parallel plates. The plates are sheared (i.e. displaced with respect to one another) whilst remaining parallel. The ease with which the plates are sheared is clearly related to the viscosity of the nematic between the plates, which in turn will depend upon the orientation of the nematic director between the plates. A variation of flow velocity will occur normal to the plate surfaces. If we consider a situation in which the nematic orientation does not alter during the shearing process, there are evidently a number of fundamental situations (see FIG. 1), each involving a viscosity coefficient, namely:
Director parallel to velocity gradient: xcex71 
Director parallel to flow direction: xcex72 
Director normal to shear plane: xcex73 
A more mathematically rigorous approach shows that we must also include a fourth viscosity, xcex712, describing a stretch-type deformation.
Finally, we must introduce a viscosity (xcex31) which describes situations in which the orientation of the nematic is not fixed, where instead a region rotates with respect to the remainder of the system.
Any dynamic motion of a nematic can be described in terms of these five viscosity coefficients. These viscosities can be experimentally determined through an experiment such as described by Ch. Gahwiller xe2x80x9cDirect Determination of the Five Independent Viscosity Coefficients of Nematic Liquid Crystalsxe2x80x9d, Molecular Crystals and Liquid Crystals, 1973, Vol. 20, pp. 301-318. Once the coefficients are known for a nematic, their use in the theory of Leslie can produce a detailed understanding of the motion and response of a nematic liquid crystal to an applied voltage.
It has been found that a particular, non-obvious, relationship between three of the five coefficients has advantage for the response of speed of the particular liquid crystal devices such as the pi-cell and the analagous device described in WO 97112275.
In xe2x80x9cThe pi-cell: A fast liquid-crystal optical switching devicexe2x80x9d, P. J. Bos and K. R. Koehler/Beranxe2x80x9d, Mol. Cryst. Liq. Cryst., 1984, Vol. 113, pp. 329-339, the authors point out that the pi-cell (a device in which the pretilt points in the same direction on both surfaces) has a speed advantage over a device with oppositely directed pretilt at both surfaces. FIG. 2 of the accompanying drawings illustrates this.
As shown in the upper row of FIG. 2, when a voltage is removed from a device with anti-parallel pretilt the subsequent relaxation of the director causes a xe2x80x9cbackwardsxe2x80x9d torque on the director in the centre of the cell. This torque slows down the cell relaxation.
As shown in the lower row of FIG. 2, in a pi-cell with parallel pretilt, no such xe2x80x9cbackwardsxe2x80x9d torque is produced in the centre of the cell and hence this device relaxes more quickly.
Bos. et. al. do not teach how a correct choice of viscosity coefficients can maximise the speed of the pi-cell. A detailed analysis has enabled us to show that maximum speed is obtained for a pi-cell when the parameter (xcex71xe2x88x92xcex72)/xcex31 has a value which departs significantly from unity.
It is known for the parameter (xcex71xe2x88x92xcex72)/xcex31 of nematic liquid crystals to have a value of 1.0 or close thereto. This parameter is theoretically predicted to have a value of 1.0 and this has been confirmed experimentally for a wide range of nematic liquid crystal materials, for example as disclosed in D. Armitage and J. Larimer, SID 96 Digest, pp. 584 (1996). It has been found that, by selecting this parameter to be substantially different from one, the switching speed and temperature response of a surface mode liquid crystal device can be varied, for example so as to provide more rapid switching speeds for video display panels or for field-sequential video display panels. Selection of this parameter also allows devices to be provided for operation over wide temperature ranges such as in-car displays and portable outdoor display devices (for example personal organisers). The prior art does not disclose the role of nematic viscosities in surface mode device switching.
The nematic liquid crystal may have an underlying smectic phase.
The liquid crystal layer may comprise a polymer network formed by polymerisation of a polymerisable material with the layer cooled to a temperature at which the nematic liquid crystal has a smectic phase.
The liquid crystal may have positive dielectric anisotropy and may be disposed between first and second alignment layers providing substantially parallel alignment and a pretilt less than 45xc2x0 The pretilt may be less than 10xc2x0.
The liquid crystal may have negative dielectric anisotropy and may be disposed between first and second alignment layers providing substantially parallel alignment and a pretilt greater than 45xc2x0, preferably greater than 80xc2x0.
(xcex71xe2x88x92xcex72)/xcex3 may be less than zero.
According to a third aspect of the invention, there is provided a display comprising a device according to the first or second aspect of the invention.